The basic measuring circuit which is designed to monitor an input signal whose magnitude varies with time is inherently limited to a specific useful range. When the performance requirement of a particular application is beyond the capability of the basic measuring circuit, auxiliary circuits are required. These additional circuits may include an input range switching circuit (manual or automatic) or a dynamic feedback circuit having a function equivalent to a continuous automatic gain control (AGC) circuit. The requirement for low noise, wide-band frequency response and good linearity, make the designs of these auxiliary circuits complex and expensive.